Electroluminescent panels form low power light-emitting displays for use in many applications. One particular area in which electroluminescent panels can be useful is in lighted signs for advertising and the like.
Electroluminescent panels make use of electroluminescent properties of certain phosphor-impregnated glasses. When an AC voltage is applied across the electroluminescent glass, the electroluminescent glass emits visible light. If an optically transmissive path is available, the emitted light travels outwardly from the electroluminescent glass where it is visible to an observer.
FIG. 1 shows one prior art electroluminescent panel 40 with several layers shown to exaggerated thickness for clarity of presentation. The electroluminescent panel 40 includes a planar metallic baseplate 42 that forms the body of the electroluminescent panel 40 and also acts as a reference electrode. A thin electroluminescent layer 44 carried by a thin bonding layer 46 covers a portion of the baseplate 42. Typically, the bonding layer 46 includes two layers, a ground coat and a white overlayer. The bonding layer 46 typically is on the order of 0.005" thick and the electroluminescent layer 44 is 0.002" thick. The electroluminescent layer 44 typically is a phosphor-impregnated glass such as a zinc sulfide doped with manganese or copper phosphor in a lead-free glass. The electroluminescent layer 44 is deposited by spraying and then firing. The bonding layer 46 is a high adhesive enamel that links the electroluminescent layer 44 to the baseplate 42 to improve the adherence of the electroluminescent layer 44.
A conductive, optically transmissive cover electrode 48 formed from an optically transmissive conductor, such as indium tin oxide (ITO), overlays the electroluminescent layer 44. Together, the baseplate 42 and cover electrode 48 form a pair of electrodes positioned on opposite sides of the electroluminescent layer 44 and bonding layer 46. When an AC voltage is applied across the baseplate 42 and cover electrode 48, an AC electric field is induced in the electroluminescent layer 44. The AC electric field causes the electroluminescent layer 44 to emit light. Some of the light passes directly through the cover electrode 48 toward an observer. Some of the light travels toward the baseplate 42 and strikes the bonding layer 46. The bonding layer 46 reflects light traveling toward the baseplate 42 back toward the cover electrode 48, because the bonding layer 46 is reflective. The reflected light then passes through the cover electrode 48 and is emitted toward an observer.
The enamel of the bonding layer 46 typically is formed from a clay containing trapped gas bubbles which are incorporated in the clay with a specific bubble structure to improve the flexibility and adherence of the bonding layer 46. The gas bubbles can affect the electrical properties of the bonding layer 46, principally by reducing the dielectric constant. The bubble structure for maximum flexibility typically differs from the bubble structure for optimum dielectric construct. Thus, the choice of bubble structure may require a significant tradeoff between durability and electrical performance.
To improve the enamel's adhesion, the enamel typically includes a metal oxide component. Unfortunately, the addition of metal oxide typically deleteriously affects electrical properties of the bonding layer 46 by increasing loss and changing the effective dielectric constant. Consequently, where metal oxides are used, it can be difficult to establish the proper electric field conditions within the electroluminescent layer 44 for proper emission of light.
Also, cracks, holes or thin spots in the electroluminescent layer 44 and bonding layer 46 can cause shorting between the cover electrode 48 and the baseplate 42. Such shorting can impair operation of the panel 40 and can pose safety hazards such as biasing the exposed rear surface of the baseplate 42 to a high voltage or drawing excessive current from a power source. To reduce the risk of cracking, pitting, or thin spots, the typical approach to adhering the enamel of the bonding layer 46 is to first abrade the baseplate 42 before coating with the bonding layer 46. However, such abrasion forms an uneven surface on the baseplate 42, thereby requiring a relatively thick bonding layer 46 to thoroughly cover the baseplate 42. This limits the minimum separation of the baseplate 42 and cover electrode 48, thereby increasing the required AC voltage for a given electric field intensity. Because the level of light emission depends upon the electric field intensity, the relatively large separation of the baseplate 42 and cover electrode 48 requires a high AC voltage. Moreover, the uneven surface of the baseplate 42 makes the thickness of the electroluminescent layer 44 difficult to control. Because the thickness of the electroluminescent layer 44 is difficult to control, the electric field within the electroluminescent layer 44 is difficult to control, making the performance of the electroluminescent panel 40 unpredictable.
To protect the cover electrode 48 and to hermetically seal the electroluminescent layer 44, an optically transmissive, insulative passivation layer 50 covers the cover electrode 48, the electroluminescent layer 44, and part of the baseplate 42. Typically, the passivation layer 50 is a high durability glass coating. The passivation layer 50 conventionally covers only one side of the baseplate 42 to allow easy electrical connection to the baseplate 42.
FIG. 2 shows a typical installation of the prior art panel 40 as an advertising sign where the cover electrode 48 is patterned to a desired shape. In this application, the baseplate 42 is bolted to a support pole 52 by a pair of bolts 54. The pole 52 is driven into the ground such that the pole 52 supports the electroluminescent panel 40. If the pole 52 is conductive, the pole 52 also electrically grounds the baseplate 42. The cover electrode 48 is connected to a cable 56 to allow a driving voltage V.sub.in to control the voltage of the cover electrode 48 with respect to ground.
Several difficulties arise with such signs. For example, as can be seen in FIG. 1, the electroluminescent layer 44 and passivation layer 50 cover a single side of the baseplate 42. If the thermal coefficient of expansion of the passivation layer 50 is different from the thermal coefficient of expansion of the baseplate 42, the different expansion rates of the materials can cause the electroluminescent panel 40 to warp in response to temperature changes.
Also, in many applications, such as in an outdoor display, the temperature swings back and forth between high and low extremes. Under such circumstances, the differential expansions of the materials can cause the panel 40 to flex repeatedly, causing premature aging of the layers 44, 46, 48, 50. Repeated temperature cycling can eventually cause cracks in the materials and cause the electroluminescent panel 40 to fail prematurely.
A further drawback of the prior art panel 40 is that the outermost electrode (the cover electrode 48) is the "hot" electrode, i.e., carries a high voltage. Thus, only the passivation layer 50 prevents the high-voltage electrode from exposure. However, any number of sources can cause gaps or cracks in the passivation layer 50. For example, the temperature cycling described above can cause the passivation layer 50 to crack and/or peel. Similarly, objects such as rocks from a nearby road can strike the passivation layer 50, causing holes and exposing the high-voltage electrode 48. Any gaps or cracks in the passivation layer 50 can expose the cover electrode 48, posing a danger of electrical shock.